1. Field of the Invention
The present invention relates to a reflecting type optical system with a reflector fixed at a rear of a transparent body with a predetermined optical relationship relative to the body, for example, such as a reflecting type telephoto lens system with a second reflector fixedly disposed behind a front lens with a spacer sandwiched therebetween. The invention also relates to a method for producing such optical system, especially for fixing the reflector at the rear of the transparent body, and masking properly the transparent body side of the reflector.
2. Description of the Prior Art
Reflecting type telephoto lens are generally so constructed that light rays from an object to be photographed, at first traverses a front lens and then strike a first reflecting surface (normally concave) which reflects the light rays forwards towards a second reflecting surface which in turn direct the light rays rearwards, i.e. towards the camera body. One example of such a telephoto lens is shown in FIG. 1, wherein the reference numeral 1 denotes a front lens of a transparent optical material behind which is fixedly mounted a mirror lens 3 having formed on the left side surface (as viewed in the Figure) a light reflecting layer 3a. While the numeral 2 denotes another mirror lens having formed thereon a light reflecting layer 2a at the off axial portion of the rear, i.e. right side surface (as viewed in the Figure), and 4 and 5 denote respectively concave and convex rear lenses cemented together. With this construction, the light ray L from an object to be photographed, after traversing front lens 1, enters mirror lens 2 to be reflected by light reflecting layer 2a (first reflector) toward mirror lens 3, of which light reflecting surface 3a (second reflector) in turn directs the light ray rearwards so that the light ray traverses the central portion of mirror lens 2 where the light reflecting layer is not formed, and further passes through lenses 4 and 5 towards a film plane of a camera (not shown).
In the above optical system, the light reflecting surface 3a at the left side surface of mirror lens 3 must be disposed precisely with respect to the axial distance from the rear surface of front lens 1. The optical member 3 having the second reflector 3a must be optically centered with the front lens 1 without inclination of the reflecting surface. To this end, a spacer is interposed or sandwiched between the front lens and the optical element having the second reflector. If such a spacer is of a transparent material, such as an optical glass without light absorptive material therein, ambient lights passing through the central portion of front lens and the transparent spacer is reflected by the second reflector or reflecting layer back to the front and then reflected or refracted at the boundaries between the spacer and front lens and between the front lens and the air outside of the lens, to become stray light which will cause ghost images in the picture and increase the flare to deteriorate the quality of the lens system. Especially, the light entering the front edge portion of the spacer can cause an extremely bad effect by the edge effect with which the edge portion glistens or glints.
As countermeasures to eliminate such extra and unnecessary reflected light, the following measures of masking have been known:
(1) The spacer is painted or coated with black paint at its front surface, i.e. the surface facing the front lens, and then cemented to the front lens. PA1 (2) The spacer is coated with an opaque layer, such as of chromium oxide or gold, by vacuum deposition or evaporation over its front surface and then cemented to the front lens. PA1 (3) The spacer is cemented to the front lens with bond having mixed therein a light absorptive material. PA1 (4) The space is made of an opaque material such as black glass.
Of the above countermeasures, the measures (1) and (2) have a disadvantage that the bonding strength between the spacer and the front lens is low or insufficient because the bonding strength between the surface of the spacer and the layer painted or coated by the vacuum deposition or evaporation is not very high. Moreover, if the material to be coated is a light reflective material such as gold in the case of (2), the light reflected by the layer will cause ghost images and flares. In the case of (3), the light absoptive material mixed in the bond reduces the bonding strength of the bond in proportion to the amount of the light absorptive material. If the amount of the material is reduced, then the bonding strength of the bond will be raised but the masking effect will be lowered.
The measure of (4) also has a disadvantage as follows. The front surface of the spacer is generally required to have the negative shape of the rear surface of the front lens, i.e. the curvature having the offset relationship with the curvature of the same, (if the rear surface of the front lens has a curvature of r.sub.o, the curvature of the front surface of the space is required to be -r.sub.o), in order that the position of the second reflecting surface relative to the other optical elements may be ensured with respect to its axial distance and of optical axes i.e. centering. To this end, the front surface of the spacer has been polished as smooth as the lens and centered and edged. However, since the spacer was usually made of glass or the like, interference of light occurred where there was a slight noncoincidence in the shape or curvature between the surfaces of the front lens and the spacer or a slight mismatching to form a wedged air film between the surfaces, so that interference patterns were observed in some of the telephoto lenses from their front. Such intereference pattern or fringes will be of concentric rings i.e. fringes of equal thickness as shown in FIG. 2(a) when the radii of the curvatures of the surfaces are slightly different from each other, while stripes of parallel dark lines i.e. fringes of equal inclination as shown in FIG. 2(b) will appear where the wedged space is formed between the surfaces. When there is a disorder in the front surface of the spacer, an irregular pattern of fringes will be observed such as FIG. 2(c). These interference patterns or fringes will be more conspicuous when the optical system with such a spacer and front lens is incorporated or assembled in a lens barrel mechanism, because the spacer and the lens are surrounded by the dark walls of the barrel mechanism. Thus the inference patterns devaluate the lens systems with respect to their commercial quality. If the lenses bearing such interference patterns or fringes are not to be put on market, then the percentage of produced products with the desired quality will decrease extremely. Furthermore, in an optical system including a transparent body to which an opaque body is attached with a polished surface of the latter being in close contact with the former, it is likely that scratches on the smoothly polished surface are more noticeable and will spoil the appearance of the lens or lens systems. There have been also known optical systems wherein a spacer of the light absorptive material is bonded to a transparent front lens with the spacer being coated and masked in accordance with one of the above mentioned measures (1) through (3). However, such optical systems still have the disadvantages inherent in these measures.
The present invention is substantially based on the measure (4) but is contemplated to provide an improved masking for the above mentioned type optical system.